(1) Field of the Invention
The present invention relates to a process for preparation of a semipermeable membrane for selective permeation and separation of a specific component in a liquid mixture. More particularly, the present invention relates to a process for preparation of a high-performance semipermeable composite membrane which can be used for desalting brackish water to obtain potable water, for removing or recovering a contaminant or useful substance from waste water causing environmental pollution, such as dyeing waste water or electrodeposition paint waste water, to enable a closed system for the treatment of waste water and for producing ultra pure water to be used for the production of semiconductors.
(2) Description of a Related Art
As the semipermeable membrane heretofore industrially used, there can be mentioned an asymmetric membrane formed of acetyl cellulose, for example, a Loeb type membrane disclosed in the specification of U.S. Pat. No. 3,133,132 or U.S. Pat. No. 3,133,137. This membrane has problems in connection with the hydrolysis resistance, the microbiological resistance and the chemical resistance, and if it is intended to improve the permeability, a membrane having a good pressure resistance and durability cannot be obtained. Accordingly, membranes of this type are not practically broadly used though they are used in much limited regions. Research works have been made with a view to developing new materials free from these defects of the asymmetric membrane of acetyl cellulose, mainly in the U.S.A. and Japan. Materials in which some defects are eliminated, such as polyamides, polyamide-hydrazides (see the specification of U.S. Pat. No. 3,567,632), polyamide acids (see Japanese examined patent publication No. 55-37282), closslinked polyamide acids (see Japanese examined patent publication No. 56-3769), polyimidazopyrrolones, polysulfonamides, polybenzimidazoles, polybenzimidazolones, and polyarylene oxides, have been developed, but their selective separating property or permeability are inferior to those of the acetyl cellulose membrane.
As a semipermeable membrane of a type different from the Loeb type membrane, a composite membrane has been developed comprising a microporous substrate and an active layer substantially controlling membrane performance, which covers the microporous substrate. In this composite membrane, materials suitable for the intended use can be selected for the active layer and microporous substrate, and the freedom of the membrane-preparation technique is increased. Furthermore, although the Loeb type membrane must be stored in the wet state, the composite membrane can said to be stored in the dry state.
This composite membrane is divided into two types, that is, a type comprising an active layer coated on a microporous substrate through a gelled layer, and a type comprising an active layer coated directly on a microporous substrate. Specific examples of the former type are disclosed in Japanese unexamined patent publication No. 49-133282, Japanese examined patent publication No. 55-38164, PB Report 80-182090, Japanese examined patent publication No. 59-27202, and Japanese unexamined patent publication No. 56-40403. It is thought that the preparation of the membrane of the former type is easier than the preparation of the membrane of the latter type and much research has been made into the membrane of the former type. However, when the membrane is used for reverse osmosis under a low pressure, the water permeability is reduced and a satisfactory performance cannot be obtained. Moreover, it is difficult to obtain a membrane having enough chlorine resistance, which is an important factor for practical application to the reverse osmosis.
As specific examples of the membrane of the latter type, there can be mentioned membranes disclosed in the specification of U.S. Pat. No. 3,744,642, the specification of U.S. Pat. No. 3,926,798, Japanese unexamined patent publication No. 55-147106, the specification of U.S. Pat. No. 4,277,344, and Japanese unexamined patent publication No. 58-24303. In the composite membrane of this type, if a high permeability is desired, since the active layer is coated very thinly, defects are readily formed by flaws or foreign substances on the microporous supporting membrane, and it is considered generally difficult to manufacture a high-performance membrane stably with good reproducibility on an industrial scale. However, most membranes having chlorine resistance, heat resistance, and chemical resistance are those of the latter type. As the chlorine-resistant membrane, a piperazine type membrane (see the specification of U.S. Pat. No. 4,129,559) has attracted attention, and especially, a high water flux composite membrane composed of piperazine crosslinked with an aromatic polyfunctional acid halide has been proposed and has also attracted attention (see, for example, National Publication No. 56-50006, the specification of U.S. Pat. No. 4,259,183, and PB Report 288387). This membrane is excellent in that the water permeability is high even under a low pressure. However, the membrane is defective in that the sodium chloride rejection is relatively low, e.g., about 50%. In the desalination process and the production of ultra pure water for the production of semiconductors, a membrane having a high sodium chloride rejection is now desired, and improvements of the piperazine type composite membrane are proposed (see, for example, Japanese unexamined patent publication No. 59-179103 and Japanese examined patent publication No. 61-27083). These improvements, however, are not satisfactory because the water permeability is reduced, and a membrane exceeding the above-mentioned piperazine type composite membrane has not been developed.